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Why might a flight have the same departure airport as its destination?
I am not very familiar with the world of air traffic.
Recently I have been granted access to OpenSky Network. As I am trying to understand the "flight" dataset, I have come across a few flights that have the same departure and destination airport, what could be the reason for this?
My initial guess is that this is for maintenance purposes, and these flights have test flights done ever so often (I'm still surprised that some of these flights are performed at major international airports however).
Could this be because the data is biased?
Any help would be appreciated!

Try to look at FR24 or other tools: the the real path of the plane, and if it was in maintenance the days before that. Data quality check requires also to cross-check with other sources.

What do you mean by "biased"? Can you give some examples of flights that show this?

Bias is a term used in statistics to refer to a dataset where errors may have leaked due to mishaps in the data collection process.

Many people think that almost all flight are passenger flight going from A to B, probably because this is the type of flight most people encounter in their daily lives. In reality, these types of flights are only part of a much more diverse reality.
General aviation/leisure/sightseeing flights will typically depart and arrive at the same airport, this often being the airport where the plane is parked in a hangar when not used.
For military (training) flights, an aircraft will also typically be based at a specific air base. Here you will also see flights departing, flying around in an exercise area, and then returning to the same airfield.
Same thing with civilian flight schools, where aspiring pilots learn to fly.
Then there's medical flights/HEMS/coast guard etc. Same deal.
As you speculate, there are maintainence and check flights as well, and aircraft certification flights. This is a screenshot of an 11 hour flight from Toulouse to Toulouse via the North Pole:

(source)
You could also have calibration flights, whose purpose it is to test and calibrate ground based navigation equipment. They will typically depart, fly various patterns around the desired navigation aids and then land again at the same airport.
Survey flights, which take the aerial photos you see on Google Maps and other services, might also depart and arrive at the same airport.
And probably many other reasons, I can't think of right now.

Flight tests for experimental equipment (either the aircraft itself, or something where you want to test how it performs when it's actually up in the air).

Not just photographic survey flights but geophysical survey flights that use various airborne instruments to sniff out resources.

Don't medical flights typically stop somewhere to pick up a patient? Or isn't that what you mean by medical flights?

@Llaves There are different types of medical flights. Some might be point-to-point (e.g. a pre-arranged trip to carry a patient from one airport to another with medical personnel on board), but you could also have a medical flight that departs its home base, picks up a patient from a field or plucks them out of the ocean or off a cliffside or wherever, and returns to the base airport where a ground ambulance takes the patient the rest of the way. You could also add law enforcement flights to this list.

Zero-G flights.

@Llaves I think there is a distinction between fixed wing medical flights that definitely go between two places and helicopters which might land somewhere. In the latter case it might be obvious in a flight tracker as a precipitous decrease in altitude at some point in the flight!

Medical rotors definitely will leave base for test flights and return. Very frequent after maintenance on the helicopter.

Helicopters used as camera platforms for sporting / cultural events / news. Helicopters and aircraft used as aerial radio relays for television signals. For example, in road cycling, you typically have cameras on motorbikes which relay their signals to helicopters, which in turn relay their signals to airplanes, which relay them back to a ground station at the TV compound near the finish line. Especially on mountain stages where both the motorcycles and the helicopters often are deep inside valleys.

Parachute/skydiving flights also tend to land right where they started from.

Three possibilities occur to me:

Not so much from the giant international airports, but most student training flights and nearly all "tour" or sightseeing flights would return to their departure airport for landing.
Also, airlines perform plenty of maintenance work at their major hubs, so when a maintenance check flight is required, such as after certain flight control replacements or changing out both engines, those would do the same thing, returning to the same airport (and probably to be put back into service shortly thereafter, assuming the checks all passed).
And, rarely, you get an aircraft that departs intending to go elsewhere, encounters a problem like bad weather at its destination or a mechanical issue, and it returns to its origin. Those would both be uncommon, and unlike the other cases, the filed destination wouldn't be the same as the departure, but after the flight is over, the two would in fact be the same.

I can also imagine training flights start/end at the same airports. I can also imagine scientific flights doing so, although hard to imagine from a commercial airport. In the same vein, I can imagine military flights (e.g. bombing/recon flights).

Christchurch (New Zealand) to Christchurch via Antarctica. It's sightseeing for people who like auroras.
Christchurch is the second biggest international airport in NZ.
16 page glossy e-brochure here
Astounding (high) prices).

Due to the brevity of this answer, please confirm that you have no relationship with the linked website or the provider.  Thanks!

I have no relationship with the linked website or the provider. This particular service lands at the same airport where it took off, does not land anywhere else, and accepts paying passengers. It seems to me to be a good example of what the OP was asking about. I have no intention of using the service, since I'm more interested in the destination than the journey.

Thanks for clarifying Simon. We get a lot of spam on SE, so just checking.  Yes, I agree, it's a good example.  Probably not the most common example, but one of the most unique for sure!

@EndAnti-SemiticHate After I had posted I remembered something about the origin of this flight, which was a response to the pandemic. What do you do with your 787s if your country has decoupled itself from the world, and you don't have enough population to fly them between cities?

That's truly fascinating, and very innovative way to keep people employed and the aircraft running without flying them around in circles with no passengers.

@EndAnti-SemiticHate Yes. Fly them around in big circles with passengers, and serve a nice meal with wine;-)

@simon IF my edit is approved - added some bulk. Amend as desired. $1500 - $8000 NZD !!!

@RussellMcMahon Thanks for added detail. As for prices, there was no competition: nobody else was flying the route and, during the pandemic, there was nowhere else to go.

@SimonCrase: These flights actually started way before the pandemic. We did similar flights from/to Whitehorse, Yukon, Canada (much closer to the Arctic circle than NZ is to the Antarctic circle, so a 3-hour flight) starting in 2017, and the guy who had the idea actually got it from hearing about those NZ flights. He had told me that NZ had stopped doing the flights, though… We stopped because of the pandemic, and he passed away meanwhile (non-COVID-related), so I’m not sure we’ll do them anymore…

@PierrePaquette  NZ used to do flights over the Antarctic proper, 1977-1979. Which lead to the Flight 901 crash into Mt Erebus. A very major event in NZ society. It was said everyone in NZ knew someone who knew someone who died. Probably close to true (I did :-( ). https://wiki2.org/en/Mount_Erebus_disaster

@RussellMcMahon That estimate sounds plausible. There were 257 fatalities; Dunbar's number is 150, so 257 * 150 * 150 is greater than the NZ population at that time.

@RussellMcMahon: Very sorry to read that, but considering it was daytime (months-long daytime in Antarctica), this was not a flight to see the aurora, but just Antarctic sightseeing (which must be very spectacular as well).

@PierrePaquette Yes, Sightseeing. I did not mean to suggest that they were aurora viewing flights. || The accident was very very much mulled over in all possible forums at the time.|| The ultimate cause was a chain of unintended events, as is very often the case in major disasters. Not enough room here for all the sub plots but. || Original intended course took them OVER. Erebus. This was NEVER implemented as a 2 figure transpostion in course data took them up Macmurdo sound. During a routine check the course was recalculated. An error in 4th decimal place was found and course was updated. ...

... this would have had a trivial effect on the course BUT the update also corrected the unknown 2 digit transposition error. This placed them back "over" Erebus. In the total Antarctic whiteout the horizon vanishes and they though they were flying up Macmurdo so safe on instruments. They weren't. || Terrain RADAR sounded but was too late. || MUCH more dastardly deeds followed as blame was passed to and fro. || I and many NZers sat at our radios/TVs for long hours as the max available return deadline approached and was passed :-(.

@RussellMcMahon Apropos "dastardly deeds", you may recall the phrase "orchestrated litany of lies" being used by the Hon. Peter Mahon, who presided over the Royal Commission into the crash.

@SimonCrase I recall it well. The whole affair was very sad and very interesting. Few seem to know that the brief details that I provided above were the foundational cause and all the rest was layered on top. Claims that eg they were flying into unknown terrain at below the known altitude of Erebus were completely true - BUT an unknown number of the other flights would have done the same and IF the navigation system worked as well as it always had on ALL other Air NZ flights then what they did "should have been safe" and was acceptable to all other. The "corrected" course data killed them :-(.


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Do rudder pedals slide or hinge?
I am planning on making a pair of rudder pedals from scratch. I don't know whether they should be hinged, or slide forwards and backwards.
On real aircraft, how do they work? Does it vary from plane to plane?

I think the OP should clarify:  Is the question relating to how the pedals adjust to different pilots (e.g., Diamond DA-42, where a lever releases the pedal mounting to slide forward and back - in this case the seat is non-adjustable, or how the pedals move (pivot or slide) when used in flight.

Interesting that most answers here seem to contradict the answer here.

I've never seen "sliding" rudder pedals in a light GA plane - though there may be such designs in transport aircraft, especially fly-by-wire planes.
If you're looking for simplicity for a homebuilt aircraft or simulator I'd go with hinged pedals pivoting at the floor similar to what you'll find on a Piper Cub (which can be made by welding tubes together into an I shape), or any number of similar floor-mounted designs:

(As shown in the first picture, toe brakes can be easily retrofitted onto such a design, and as shown in the second you can mount square pedals on them if you want to, though the round bar has some advantages.)

"I've never seen "sliding" rudder pedals in a light GA plane - though there may be such designs in transport aircraft, especially fly-by-wire planes."
What GA planes do you base your statement on (just curious)?

@flyingfisch Mostly Pipers & Cessnas, a few LSAs that I've had occasion to climb into as well.  They've all had some kind of pivot mechanism (either a simple pivot like above or torque tubes) rather than a track that the pedal slides in. I could certainly see designing a sliding rudder pedal though - it would be simple enough to make it work with cables, and even easier with an encoder for fly-by-wire aircraft.

yeah, I guess it would be easier to build sliding pedals than pivoting ones...

I know some Extras have "electrically adjustable rudder pedals", but I don't know how they operate.

@DanPichelman I'm not sure about the Extras but the Pipistrel Virus has a pretty awesome setup for (manually) adjustable pedal distance: https://www.youtube.com/watch?v=NfgllFkauZY

@voretaq7; that's virtually identical to how you adjust the pedals in the Diamond Katana. It's quick and not very error prone, I like it.

Sliding pedals are very unusual - I do not know of a single aircraft which uses them. It is also mechanically more complex and fragile - you would need to keep all those sliding lines clean, at a section of the aircraft where dirt is most common.
But there are three different ways to hinge the pedals. The most common one is at the bottom, but they could also be mounted on a crossbar. By using two crossbars, you get a parallelogram wich helps to keep the pedals at the same angle.

This picture shows the "cockpit" of the schooling glider SG-38, simply because it doesn't hide anything. As you can see, the pedals sit at the ends of the parallelogram, and you could add another hinge at their base easily for activating brakes (which the SG-38 did not have). Now you would have two degrees of freedom which make controlling two separate functions easy.
The third way is to hinge the pedals from the top. This again helps to add a second degree of freedom, especially when the top hinged mechanism uses two parallel rods to again ensure a constant angle of the pedals, independent of their position relative to the hinge point.

On the Cessna's I've flown they both slide and pivot- sliding directs the rudder, pivoting engages the respective wheel's brakes.

What Cessna models?

Cessna 172s, from a range of model years.

OK, maybe it varies from manufacturer to manufacturer then.

@CoderTao All the Cessnas I've been in have had rudder attached to torque tubes (built something like this) - I consider that a pivot (around the mounts on the floor) rather than a slide (in a track). Is this what you're describing, or have you seen something different? (Incidentally side-by-side Pipers have similar rudder pedal designs, but with the torque tube above the pedals instead of below them - you can sometimes find full rudder pedal assemblies for both Piper & Cessna on eBay)

I can't comment on the actual construction / physical implementation- I never looked that close at them (best I can say is they're similar to what's seen here: http://farm3.staticflickr.com/2724/4337838025_18c852a8f2_o.jpg ); only that there were two actuating motions, one which seemed to be a linear slide, and the other a pivot.

All 100 and 200 series Cessnas that I have worked on have very similar assemblies, including many of the same part numbers.

The gliders I've flown (Blanik L-23 and Grob 103) have hinged pedals with regards to controlling the rudder, and they slide closer or farther in tandem to adjust for different pilots' heights.

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Is there a difference in stability between a tail downforce (cg ahead of ac) and a tail upforce (cg behind)?
So from what I understand, having a -
1.cg ahead of the neutral point(ac) with a tail downforce
and
2.cg behind ac with a tail upforce
are both stable configurations.(Please correct me if I'm wrong.)
Will the first configuration be more stable due to less lift at the tail?
What are the advantages and disadvantages of both these configuration?
Also if I have a configuration with cg between cp and neutral point, neutral point being the most aft point, will it be stable?

No, cg ahead of neutral point is stable, cg behind it is unstable.
Neutral point is the centre of pressure when both main wing and horizontal stabilizer fly at the same $C_L$. If the cg is just a bit ahead of neutral point, the tail still flies with positive lift. If it is ahead of the centre of pressure of the main wing, tail flies with negative lift.
The further forward the centre of gravity is, the bigger the difference between the coefficients of lift and therefore the stronger restoring force. But note that since it is second order negative feedback, the restoring force creates phugoid oscillation and if that is too fast, it will be difficult for the pilot to stop. So the stability has to be just right.
This is still true in canard configuration, but there the forward surface that flies at higher $C_L$ in stable configuration is the canard.

When you say neutral point do you mean the aerodynamic center of the entire aircraft?

@Zaber, I mean aerodynamic center of the entire aircraft at a very specific condition when both main wing and horizontal stabilizer fly at the same coefficient of lift—which is the point where putting the cg will result in neutral stability. Because aerodynamic center of the entire aircraft moves with the elevator position otherwise.

(@Zaber, and wikipedia confirms that that is indeed the correct terminology)

Am I correct in saying that the tail will fly with a negative lift when cg is ahead of the wing aerodynamic center?

@Zaber, yes, you are. But it is mostly irrelevant for longitudinal stability, which requires that the aft airfoil flies at lower coefficient of lift than the forward airfoil, but it does not have to be negative (of course when it is negative, the difference is bigger and that means the restoring force is stronger).

How does the aerodynamic center move with elevator deflection? Isn't ac assumed constant at 25% chord? What does ac movement depend upon?

@Zaber, the centre of pressure of the whole aircraft does. The aerodynamic centre of the main wing does not (but does with with flap extension—it is not exactly 25% chord, it is approximately 25% and depends on the profile)

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In an Airbus A-320, when is it appropriate to use: vertical speed mode, the expedite button, and the speedbrake?
When it is appropriate and good airmanship to use Vertical speed on A-320 and where it is written?
Also same questions for the speedbrake?
Also, why do some captains call the expedite push button the "killer button"?

Some of these answers really depend on specific situations and pilots are trained to decide. Moreover, each airline defines Standard Operating Procedures (SOP) which may include some rules. But let's look at what the Flight Crew Training Manual (FCTM) of A320 says about them.

When it is appropriate and good airmanship to use Vertical speed on A-320 and where it is written?

It suggest to use V/S when (but not only for):

High traffic to use small V/S values and avoid TCAS warnings
Small altitude changes, because pilots can select a low value for the V/S and have a smoother climb/descent
If the pilots want to change the descent path (this is really open)
When you need to intercept glideslope from above
Various abnormal situations

Also same questions for the speedbrake?

Pilots use them when the want to increase the descent rate. Its use is quite frequent and in normal condition depend on the approach or due to ATC requests. Interestingly, the A320 display, when the FMGC calculates that you won't be able to meet an altitude constraint in your planned route, it suggests you to use them with the "MORE DRAG" message:

Also, why do some captains call the expedite push button the "killer button"?

The expidite push button configures the autopilot to perform the maximum climb or maximum descent possible within the limits of the aicraft. To be more precise:

In climbing, it targets the highest possible vertical speed that keep the speed to the Green Dot speed, with the throttle in CLB mode.
In descending, it targets the lowest possible vertical speed that keep a maximum of 340 kts or 0.8 Mach, with the thottle in IDLE mode.

It also disregards speed and altitude constraints of planned flight route.
It is not frequently used because not fuel efficient and not confortable for the passengers.

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Why couldn't Air Moorea Flight 1121 use pitch trim to land safely?
On August 9th, 2007 an Air Moorea Twin Otter crashed into the ocean. The cause of the crash was determined to be loss of pitch control after an elevator cable snapped.
In my world (Cessna 152), if an elevator control broke I would be pretty sad, but the pitch trim tabs would probably allow me to put the plane down safely somewhere.
As far as I could tell based on my research, the Twin Otter is equipped with a pitch trim tab (aircraft are required to have them, but they can come in a variety of forms). Why couldn't the pilot use pitch trim to descend safely?
--EDIT--
In my checklists, I am required to "set trim for takeoff" in the engine run up. What is the point of this step if that trim setting doesn't hold the proper attitude for not falling out of the sky?
--EDIT #2--
In a highly hypothetical follow up question, would it have been possible, with 400' of altitude, for the pilot to go so hard nose down that he could pull out in an inverted configuration and fly the plane safely that way (the only cable that snapped was the elevator-down cable)?

Something that hasn't been mentioned yet, there is a step in my post-landing checklist for C152/C172 to reset the trim. When one is landing one exerts backpressure on the yoke in the flare and roundout and some may trim that backpressure out. If this checklist item is missed, the "set trim for takeoff" will take out out this excessive trim.

All quotes in this answer are from the English translation of the BEA accident report.
The elevator cable broke at "an altitude estimated at between three and four hundred feet",(p. 7) and there were eleven seconds between that time and the point of impact. If it had happened higher, the pilot would potentially have had time to figure out what happened and pull out of the dive with trim. Taken by surprise and not knowing what happened, the pilot simply did not have time to recover.
The Twin Otter would have been demonstrated to be recoverable from an elevator cable break as part of its certification (p. 17):

The CAR 3 regulation, basis of certification for the DHC6, requires a flight test demonstrating the aeroplane’s capacity to land using only the elevator trim in the case of failure of the primary longitudinal control of the aeroplane.

However, there's a big difference between "can be landed using just trim" and "can be recovered by a pilot who is surprised by the emergency". Flight tests performed during the investigation showed (p. 63, emphasis mine):

At [the height the incident aircraft was at], only immediate action on the trim located on the centre pedestal would make it possible to recover the airplane. The test also showed that from level flight it takes about three seconds for a pilot trained for and prepared for this exercise to recover the airplane.

Unfortunately, on this flight there simply was not enough time to discover the problem, figure out what went wrong, and recover (p. 63):

The highly dynamic nature of the events following the failure must be emphasized here. Eleven seconds passed between the pilot’s exclamation and the impact. This only left a short time for the pilot to analyse the situation and apply a solution that he had to improvise. In addition, the stress associated with the airplane’s attitude and the difficulty in estimating his height, in the conditions on the day, in relation to the surface of the water, certainly affected his powers of analysis. The pilot was not trained or prepared, either during his training or during type rating, as indeed most pilots aren’t, to react to a loss of pitch control. Only a reflex action could thus have
  allowed him to recover the airplane before the impact.

To answer the edits to the question, note that the airplane was not trimmed to "fall out of the sky". The pilot called out the trim checklist item before flight(p. 17), and so should have set it to the takeoff setting. Presumably this would require gentle back-pressure for rotation and climb. The event that initiated the failure process was retracting the flaps(p. 39). This changed the flight dynamics, and therefore the elevator control loads. The flight testing discovered that this created a "pitch-down moment"(p. 39):

To counter this effect and maintain the aeroplane on its initial trajectory, it was necessary to exert considerable pitch-up effort on the elevator control.

This is not a cause for concern. The procedure is to hold pressure on the controls and adjust trim to relieve the pressure. However, it was at this point that the elevator cable snapped, so the pilot was interrupted and didn't complete the process. If the incident had happened at a higher altitude, the airplane would have recovered itself from the upset (p. 42):

[S]tick free, the flap retraction led to a pitch-down moment varying the aeroplane
  trim between 20° and 30°, the rate-of-climb indicator going to stop at less than
  3,000 ft/min and speed reaching 140 kt in twenty seconds, then the aeroplane
  recovered on its own in level flight in five seconds, the total loss in height being
  seven hundred feet[.]

To the last question, flying inverted would not be necessary for recovery. I do not know the maneuvering characteristics of the DH-6, but I am unsure that an outside loop could successfully be completed starting at 300-400 AGL.

Thanks for the awesome answer! I edited the question because I am also curious about why we "set trim for takeoff" if that trim setting doesn't hold a level climb.

@BrianWheeler: I don't know for sure, but I think the point of take off trim is to avoid the nose lifting off prematurely. And since you can't feel your C/G on the ground, it has to be somewhat down trim. You are supposed to retrim after take off when you can set the trim according the actual balance.

@BrianWheeler according to the "aircrash investigation" episode I saw the pilot just pulled in the flaps which pitched the plane down and when he pulled on the yoke to level out the cable snapped

Answer edited. See if that answers your curiosity!

@BrianWheeler it'd hold a climb maybe, but not if the other means for pitch control fail. In this case the elevator snapped to maximum descent deflection, more than compensating for the up-trim.

In my checklists, I am required to "set trim for takeoff" in the engine run up. What is the point of this step if that trim setting doesn't hold the proper attitude for not falling out of the sky?

A major cause of accidents on take-off and landing is a stall followed by a spin. A wing stalls when it reaches the critical angle of attack, which is much easier to do at the lower speeds required on take-off and landing. If you set a nose-up trim you will need to use forward-pressure to keep the aircraft at a safe airspeed, if you then took your hands off the controls you could find yourself at stall speed in a matter of seconds.
It's better to require a bit of back-pressure to gain altitude, at least that way if you let go you will gain, rather than lose airspeed. Once you have an established climb speed and stable pitch attitude you can then trim for the climb.
Here's an example of how this can save your life. Imagine you are in your Cessna 152, 300 feet agl in a climb. Suddenly your seat rolls back all the way due to a broken seat lock (it's happened to me in a 152). It's much easier to put forward pressure on the controls in that situation than back pressure, and a bit of nose down trim would cancel out the sudden backward move of your C of G somewhat. Having nose up trim in that situation could potentially put you in a non-recoverable situation.
So take-off trim is actually designed to keep you from falling out of the sky.

The primary purpose of the trim tab in every airplane I've ever flown or heard of is to minimize the need to provide control input. It doesn't do anything more than aerodynamically put the elevator (or other control surface) in the position that it needs to be in for whatever flight demands there currently are. You can "fly" an airplane with trim tabs. But you're really just using the trim tabs to reposition the actual control surfaces.
The Takeoff setting on a small aircraft is usually a fairly neutral position. But it changes with conditions on larger aircraft.

Adding to this answer: "In my checklists, I am required to "set trim for takeoff" in the engine run up. What is the point of this step if that trim setting doesn't hold the proper attitude for not falling out of the sky?" A second good reason for appropriate mechanical trim at all times is that it takes stress off the cables/actuators which will reduce the chances of breakage.

Re "Suddenly your seat rolls back all the way due to a broken seat lock (it's happened to me in a 152). It's much easier to put forward pressure on the controls in that situation than back pressure, "  -- why?  Wouldn't you tend to pull the control yoke aft as you go sliding backwards?  Once you stop sliding, why is it easier to push the yoke forward than to pull it aft?  The former would seem to require longer arm reach than the latter.

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Why is the PIC position for helicopters the right seat, rather than the left seat as in airplanes?
So far as I am aware, in a fixed wing aircraft having side-by-side seating and  dual controls but only a single set of instruments, those instruments are always positioned in front of the left seat. Even when an aircraft (such as commercial air transport) is fitted with dual instruments, the left seat is designated as the "pilot-in-command" or "captain's" position. This "left-seat as principal" designation appears to hold true for any fixed wing aircraft from a Cessna 152 to a Boeing 747, in all jurisdictions of manufacture and operation.
For a helicopter, the situation appears to be reversed - the "pilot in command" appears to always occupy the right seat.
Is this true, always, or is it an observational bias? Is this simply convention or is it specified by regulation? What would account for exceptions, if any?

There are exceptions. The EC 130 has three seats in front: the PIC occupies the left seat and the copilot (if there is one) the middle seat. I don't know the reason for this arrangement. Also, the Ka-26 (an possibly other russian helicopters) has the PIC position on the left seat. As a helicopter pilot flying from the right seat makes sense, because I can keep my right hand on the cyclic all the time and I operate any other equipment with my left hand.

Interesting that this just caught my eye. I just finished watching A Chopper is Born about kit-building a RotorWay Exec 162F. Every shot of that helicopter, from the demo flights at the factory to the test pilot giving the finished kit it's initial shakedown flight, shows the PIC on the left.

@Emil When you mentioned the EC-130, were you referring to the C-130 Hercules cargo aircraft? If so, the third seat is the flight engineer's position, and the copilot sits in the right seat. In the Air Force, the flight engineer is not a pilot, and that position's job is to monitor engine functions.

@HowardMiller No, I meant the Eurocopter EC130 (which now it's called Airbus Helicopters H 130)

@Emil Thanks for clearing that up for me.

The reason is both historical and operational.
The first mass produced helicopter was the Sikorsky R-4.  It had a single collective located between the two pilot seats, whereas the cyclic was controlled by two sticks between each of the two pilots' legs, so by necessity, the person on the right would control the cyclic with their right hand, and the person on the left would control the cyclic with their left (because their right is needed on the collective).  The cyclic in the R-4 was very difficult to manipulate and required a lot of strength and coordination.  Since the vast majority of student helicopter pilots were right handed, they sat in the right seat, allowing them to use their dominant hand on the cyclic.  Thus, the first generation of helicopter pilots all sat in the right seat, since they all learned on the R-4 (or the R-6).  That's how traditions begin.
The operational applications of sitting in the right seat relate to the inherent instability of a helicopter. A helicopter pilot rarely wants to let go of the cyclic, as it often requires continuous control inputs, especially when hovering. The collective, on the other hand, does not need to be monitored as closely and can be held steady with a friction adjustment, so that hand is occasionally free to manipulate things like radios and other equipment, which are located in the center of the cockpit.
Most helicopters these days have a collective for each seat, located on the left, so the pilot wants to use the hand in the middle of the cockpit to work the radios and things.  Hence, they sit on the right side, which places the collective hand in the middle of the cockpit.
You can read a longer article about this: Why do helicopter pilots sit in the right seat?

The person on the right would control cyclic (in the middle) with their right? I seem to be missing something.

@Jan Hudec: The left hand controlled the collective.  The collective is always (as far as I know) between the seats.

@SkipMiller: So it's really the same as aircraft that have power in the middle and the outer hand always holds the flight controls. Except controlling aircraft is easier, so noone appears to mind controlling it with left hand (including monsters like A380 which has single-handed controls on the outer, so captain's left, hand).

Editing my comment above is timed out, so: Comment Continues...or to the left of the seat in aircraft with more than one collective control.  Thanks to Ralgha. I have not seen a dual collective helicopter.

@JanHudec well the A380 is fly-by-wire without feedback so it's not really monstrous

@SkipMiller every modern heli has dual collectives

@DavidRicherby: No, students always sit in the captain's seat, i.e. right in rotorcraft.

@JanHudec the important point you're missing is that the fly by wire joystick on an airbus is electronic and requires no force (not, particularly, precision), whereas the collective on the early helicopters needed a fair bit of strength, plus constant attention and accuracy. The Airbus requires little more effort to fly with the left hand than the right, especially after a little practice (similar to how I'm typing this response with both hands).

@JonStory: I never said anything to the contrary.

The thrust from the tail rotor on a counter-clockwise spinning helicopter tends to create a translational shift to the right which is countered by rigging the cyclic neutral position to be slightly left of centre. This results in the left skid "hanging low".  So as not to make this skid even lower, it is a practical reason to sit a solo occupant on the right.  Even when solo, the left skid still hangs visibly lower.  Some light helicopters are prohibited from solo flight in the left seat (e.g. R22) as it would be possible to run out of cyclic authority in some circumstances.
Here is the lateral CG chart for the R-22. As you can see, the right side of the helicopter has a greater lateral CG envelope than the left, which is why the solo pilot always sits on the right.

The opposite is true for clockwise spinning main rotors.

French helicopters have clockwise spinning main rotors and most of them still have the PIC on the right seat.

@Emil Why the PIC would still sight on the right, I do not know (can you provide an example?) but the skid low tendency would be on the right.

All French-built Eurocopters, except the 130 have the PIC on the right side (including older Aerospatiale Dauphin). Indeed, the skid low tendency is on the right, so I suppose there are more important reasons to keep the PIC on the right side than adjusting the CG position.

@Simon this is a very good answer. many FWers don't realize that helis have a lateral CG. the R22, being so light, is particularly sensitive to lateral CG, as you rightly described, and indeed has a minimum solo pilot weight as well. http://www.robinsonheli.com/manuals/r22_poh/r22_poh_2.pdf

(continuation...) its not clear, however, if this is cause or effect, in the sense that rotors go CCW historically and the radios are in the middle, so the CG envelope was designed for a right-seat solo pilot.

@rbp Thanks for the edit and the vote.

@rbp.  I've just realised the significance of your comment about the instrument binaccle in the centre.  You should add that as an answer. I don't know any helicopter designers to confirm (:)) but this would be a very strong reason to put the PIC on the right.

@simon done.com !

Short answer:
In a helicopter, because the right hand is stronger and more accurate for many people, and the cyclic control is the most demanding, the right hand is permanently dedicated to the cyclic control. Other actions have to be conducted with the left hand.
If there are many switches and controls to be reached by the pilot with his/her left hand, the most logical place to seat is on the right seat.
Bonus: If we want to remove one collective lever in a dual control rotorcraft, we must keep the right one, and then this looks like this when piloting from the left seat:

Operating a Sikorsky XR-4 with hands reversed - 1942. Source
Not something your want when you're a student. So in 1942 the student will use the right seat, and takes the habit to seat on this side.

Single control
The stick (the cyclic pitch control) is used with the right hand, the most agile one for a right-handed pilot. This control is of prime importance as a helicopter is inherently unstable requiring a constant adjustment of its attitude by the pilot using the cyclic.
The vertical speed is adjusted with the collective pitch control and the left hand. This hand also operates all other controls required during the flight, as the collective doesn't require a permanent adjustment.
For helicopters of the WW2 era which can be flown from one seat only, this seat was on the left. Typical arrangement:

Bell H-13 Sioux (Bell 47) operations during Korean War, ca 1952. Sources: Left, right
The pilot seat is on the left side, like on any aircraft. Helicopters of that era had cockpits not as complex as today.
Dual control
With a dual control helicopter, two seats are fitted with the same controls which are mechanically linked. The pilot can use any seat depending on their preference.
As helicopters were getting more complex, the number of controls and switches also increased and they couldn't be all located at a convenient place to be reached by the left hand from the left seat. Obviously from the right seat the left hand would reach more remote elements and more easily. A rationalization was necessary.
Part of it was to not duplicate all buttons, switches and controls to simplify the panels and to save weight. The non duplicated elements were moved between the two pilots, either on the vertical panel, or between the seats (on some models the middle third seat was removed too).

Sikorsky H-19 (Youtube)
Sikorsky R-4 trainer
If this wasn't enough to select the right seat and leave the left one to the fixed wing aircraft, there was a huge engine problem with the Sikorsky R-4 (the first civil mass-manufactured helicopter):

The R-4 was intended as a trainer, but was so underpowered that
Sikorsky was looking for any potential savings, so Igor and his
engineers decided to let the instructor and student share a single
collective. The only place to put it then was in the middle between
the two seats. Given the coordination and strength required to
manipulate an R-4 cyclic for any length of time, the student always
flew from the right

Source
Sikorsky HNS-1 CG (YR-4B):

Source
Even if other helicopters after the R-4, and still today have two collective controls, the habit of flying the first trainer from the right seat was sufficient to question returning to the left seat, the right seat allowing more freedom to the left hand.
And there is no need to fight with swords in our era :-)

Your question mentions PIC place. Pilot In Command is not synonymous to pilot flying (PF). PIC means the pilot legally responsible on board. PF means the person who flies the aircraft, pilots may alternate this latter role during the flight. The answer pertains to PF.

This cavalry theory might explain why airplanes were 'mounted' from the left side (although to me the naval legacy is more plausible here), but hardly the seat arrangement. Nearly all airplanes of that era (that had more than one seat) had tandem seating rather than side-by-side.

In the early years when helicopters were under powered and their rotor systems were simple and basic it made sense for the pilot to sit on the right because the advancing blade, on the right create more lift then the retreating blade on the left since the forward speed of the chopper is added to the rotational speed of the blade. This caused asymmetrical lift and it was partially compensated for by having the pilot sit on that side of the helicopter.   When flying The H 13 in Korea with both seats occupied we always placed the litter on the right skids for that reason.
  Ren

Welcome to aviation.SE!

Asymmetrical lift would roll the helicopter over, the left skid low flight would be caused by the tail rotor being lower that the main rotor. But that's a minor detail, welcome to the site.

Another reason helicopter pilots sit on the right is because the cyclic is held in the right hand, and the collective in the left.
Its safer for the pilot to take his left hand off the collective to operate the radios.

Is that a single, common cyclic in the center, instead of separate sticks joined at foot level?

its not any different than any other mechanical heli, except this one is an "outie" (visible) versusu an "innie" (invisible, under the floor boards):
http://incolor.inetnebr.com/iceman/data/fltcls2.jpg


text:
Why might American Airlines fly a 767 from JFK to EWR?
I was browsing FlightAware and saw the following:

This is an American 767 flying from JFK to Newark (EWR).  Surely this isn't a passenger flight...this isn't one of American's normal routes.  Why might an AA 767 be making this flight?

Its listed as one on American Airlines website, Flight 9743, 40 minute flight JFK to EWR, if it were a positioning or charter flight I don't believe it would appear on a search, FlightAware may have the aircraft type wrong though, its known to happen.

I looked up AA flights from KJKF to KEWK. To do that on AA, you have to do KJFK=>KBWI=>KPHI=>KEWR

I can't find it on ITA Matrix, so most likely it's going to be a repositioning flight. Since it's not bookable, and only appears to have happened once, the purpose of the flight was most likely to serve as a replacement aircraft for one that went out of service at Newark for unscheduled maintenance.

https://en.wikipedia.org/wiki/Flight_number#Number_of_digits suggests that 9xxx codes are usually positioning/ferry flights. I have no idea how generally applicable that is, but it does seem to be consistent with the circumstances here.

@Andrew I saw that statement as well. It's unsourced, and I was able to find "regular" codeshares using 9xxx flight numbers with little difficulty. So I'm sure it's not true in all cases.

It's a positioning ferry, to move the aircraft from where it "is" to where it "needs to be" for its next revenue leg (or maintenance, in some cases). These sorts of legs are uncommon because they're expensive; ideally an aircraft is never out of position this way, but sometimes "stuff happens" and you fly an empty plane to get it where it needs to be.

This sounds like a reasonable explanation. Weather can be a reason in such scenarios when an airplane is diverted to close by airports.

But why fly an empty plane. Why not service the route with some extra passengers. The only reason I can think, that it was a last minute decision.

@Firee Normally the airline would do that - they tend to plan their network so that each aircraft ends the day in the place it needs to start the next one. Sometimes though (weather, maintenance, staff issues, cancelled flights) the aircraft is in the wrong place and they have to just send it where it needs to be on fairly short notice. They might know about it a few weeks before or only an hour or two, but that's not long enough to arrange and market a commercial flight.

@Firee: You only need ferry pilots for an empty plane, no cabin crew. And with an unscheduled one-way flight, you have to find a return flight for the entire crew so it makes sense to fly with the smallest crew possible.


text:
What is the most carbon-efficient way to take off in a passenger plane?
Consider two possible flight paths for a jet passenger plane; the first is a slow climb, held low over 20 or so miles to avoid other flight paths, the second a more rapid climb over 10 miles to the capacity of the plane rather than restricted.
It has been said to me that the more rapid climb will reduce CO2.
But logic to me is that the rapid height gain will burn more fuel and so increase CO2 production (or perhaps are the 2 not linked I wonder).
I would appreciate being put out of my mystery on this?

Short answer: the higher you go (up to a comfortable minimum distance between critical mach and stall) the less fuel you will burn. The sooner you can get there, the better, but there are often other considerations.

Whilst fuel burn at higher altitudes is lower is it too simplistic to compare it to putting your foot down in a car to reach 60 in 8 seconds as appose to taking 20 seconds with a lighter touch to the accelerator? Also are noise levels not higher by pushing to a higher altitude quicker.

staying low lets the noise pollute a larger area (leading to more complaints)

@Andy The overriding factor, both for fuel burn and noise, is altitude, although I lack the physics background to understand the details of why much less than being able to explain it to others. The fuel burn lessens dramatically with altitude. Using a 747-200 as an example, the fuel burn for each engine on short final for landing will be about 5,000 lbs per hour. That's approximately the same fuel burn as at 39,000 in cruise at mach 0.86. Further, at least as I remember (it's been 15 years), the fuel burn on takeoff after liftoff is around 4 times that.

@Terry: The fuel burn at takeoff is so much higher because you create more thrust than needed for sustained flight. The excess is used for acceleration and climbing. Also, if you fly fast and low, you are far from the speed of minimum drag, so your plane flies less efficiently, consuming more fuel. This is possible at all because the engines have four times as much thrust available at sea level than in 40.000 ft.

@Terry To expand a bit on what Peter said, the air is much less dense at 40,000 ft than at sea level (as I'm sure you're aware.) Form drag is (roughly) directly proportional to air density, so the form drag for a given speed is much less at altitude than at sea level.

@Andy: If you want to compare it to a car you can't really do it with a normal car. Instead, imagine a car that burns more fuel at 10-30mph than at 60mph. Thus, while you're pressing the accelerator less, you burn more fuel the longer you take to get to 60. You basically need to balance how much accelerator you're willing to press (burning a lot of fuel) vs how long are you willing to stay under 30mph (potentially burning even more than "a lot" of fuel)

You are right, climbing adds to the fuel consumed, and increases the energy which is added to the plane, but this energy is conserved and can be reclaimed later, on descent. There are two kinds of energy to consider, potential and kinetic energy. Potential energy is weight times height, and kinetic energy is mass times speed squared, and both can be interchanged.
$$E = \frac{m}{2}\cdot v^2 + m \cdot g \cdot h$$
Nomenclature:
    $E \:\:\:$ energy
    $m \:\:\:$ mass
    $v \:\:\:\:$ speed
    $g \:\:\:\:$ gravitational constant
    $h \:\:\:\:$ height
The thrust of the engines over what is needed to overcome drag, accumulated over time, builds up energy. Lift and Drag, in turn, are proportional to airspeed squared times density of air, so if the airplane climbs, it needs to fly faster to compensate for the thinner air, and drag will stay roughly constant. Thrust is also proportional to the density of the air around the aircraft, and the same goes for fuel consumption. This means that the aircraft will approximately consume the same amount of fuel over time when it flies low and slow, and when it flies high and fast. Since the task is to transport people or goods to a specific destination, flying high and fast will result in less running time at the same fuel flow. This is the reason why flying high results in less CO2 produced!
When the aircraft comes close to its destination, it can now put the potential energy which has been stored as altitude to good use: In a descent, the thrust generation can be reduced by the amount of energy which is now reclaimed, just as you can ease off the gas pedal in your car when coasting downhill. So for the last half hour of the flight, the aircraft will now consume less fuel than before, and by the time it has reached its destination, the difference is just as big as the excess in consumption which was needed during climb.
For the nitpickers: The aircraft will weigh less at the end of the trip (fuel has been consumed), so the descent will not completely compensate for the climb. On the other hand, the lower temperature at altitude will reduce fuel consumption by letting the engines run more efficiently, so in the big picture a definite gain remains.

Thanks for all these comments. I understand what is being said about the journey as a whole but am really focussing on that take off process. From above, comparing a slow 20 mile climb to a short 10 mile climb to say 7000 feet. Are we saying that the slow climb produces more CO2 and consumes more fuel in this specific period. Also the noise generated over the rapid 10 mile height gain must be higher than over the same 10 mile distance as a slow climb? Thanks for the expert opinion being offered, really useful in helping me understand the issues

CO2 is mainly proportional to engine running time. Now if you compare a 10 mile climb at full thrust to a 20 mile climb at 80% thrust, still the runnig time will dominate the result. If we use Terry's 747 as an example, it will just stay aloft with three of the four engines running. This means you need to produce somewhat above 75% maximum thrust to climb slowly. Climbing more quickly will always be more efficient because it needs less time, even though the CO2 output per minute is higher than in the slow climb. Noise is also lower, simply because the plane is farther from the ground.

@Andy: The point is that no matter whether you have a long shallow climb or a short steep one, the energy you put into elevating the plane's mass to, say, 10 km is the same. Your spending that energy is compressed into a shorter time for the steep climb, but the total energy it costs to get to a higher altitude is the same. The difference is only in the amount of aerodymanic drag you additionally have to overcome, which is significantly smaller at greater altitudes.

From looking at emissions data, most engines indeed have very similar CO indices for takeoff vs. climb out, but on some engines one mode is much larger than the other.


text:
What benefit is provided by an MCA that is not already provided by an MEA?
I continue to read definitions of MEA as the minimum altitude across the entire segment to ensure obstacle clearance and navigation/comm signal reception. It also states that this is the lowest altitude across the entire segment.
The way I interpret this is that as I fly towards the segment I must be at or above the MEA when I enter the segment to ensure that minimum across the entire segment. Given this why is there a need for MCA (minimum crossing altitude)? The MCA definition requires that minimum altitude at the segment boundary. Why would the MEA not be sufficient?

MCA is for the fix and specifies the minimal altitude at which a normal climb can clear all obstacles.
MEA is the for between fixes and assures acceptable navigation signal coverage.

source wikipedia.
The MCA can be lower than the MEA of the adjoining route so MEA is required so you won't crash into the next mountain and MEA can also be lower than MCA so it's required so that the pilot can know to climb before reaching the fix so he knows he can get over the next obstacle safely without needing to circle or zigzag to buy time for the climb.

If you cross a fix onto a new segment/route with a higher MEA, normally you are not required to start climbing until after crossing the fix. Note that you must climb at the standard rate of 200ft/nm (or better) in this case.
If the fix has an MCA, though, you must climb  before the fix so that you cross it at or above the MCA. There is no minimum climb rate.
Typically, you will find an MCA where routes intersect near rapidly rising terrain and the standard climb rate won’t keep you safe in one particular direction away from the intersection.


text:
How can I find aircraft and airline listing data for planespotting?
I vaguely remember when younger a spotter I knew having a book listing all airlines and all aircrafts within the airline at time of print. Is this data available now in book or online format? I'm looking to get back into spotting.

the book was most likely jp fleets which unfortunately has stopped being printed. They have an online version but it  requires a subscription which is pricey

https://www.flightaware.com/ might be a good choice also.

You can find much of this sort of information at http://www.airfleets.net
For example, searching for British Airways lists their fleet, and clicking the number of active for a model (eg A318) will list the tail number and other interesting information. From there you can click a specific airplane for all the information along with a picture.

airfleets.net has details of aircraft with most airlines online and also in paper and ebook formats.
Aircraft data is also available with planespotters.net and also with flightradar24.com.

You may wish to try PlaneLogger, which is an online site for logging your aircraft sightings. It has a large database similar to AirFleets, but allows you to mark off the aircraft as you seen them.


text:
How does aileron reversal work?
I want to know how aileron reversal takes place when control reversal velocity is higher than divergence speed  and operating speed is in between them (for conventional wing)

I don't understand your question. You have a condition in your question that states "the operating speed is in between [the control reversal velocity and the divergence speed], with the divergence speed being the lower bound. If you operate an aircraft above the divergence speed, the wing will have a structural failure.

@DeltaLima: Isn't the divergence speed the speed at which the drag of the wing starts to rise steeply due to transonic effects?

Control reversal speed is way lower than divergence speed. It may actually be lower than operating speed for some aircraft, which is why they have inboard ailerons/flaperons and/or use spoilers (that don't suffer control reversal) for roll control at high speeds.
Control reversal (at high speed; there is also control reversal at stall, which is different) occurs because the force generated by ailerons twists the wing sufficiently so that the aileron starts to act more like a trim tab. For that the wing has to flex, but it does not have to flutter at all.
Spoilers act closer to the wing mid-chord, so they don't twist it much and thus don't suffer reversal. In fact they twist the wing a bit, because pre-stall lift acts more forward than post-stall lift, but this twists the wing forward and thus further reduces the lift, so still no control reversal.

As I understand it, aileron reversal can occur in two regimes of flight, high speed and near stall.
Near stall, the wing is very near it's critical angle of attack. When the pilot inputs let's say a left turn, the right aileron will deflect downward. This downward deflection increases the AOA of the outboard portion of the right wing such that the lift generated by that outboard portion is lower (drag higher as well) than the lift generated on the opposite wing.
You can kind-of visualize this by thinking of a wing operating on the backside of the lift curve.
Aileron reversal at high speed, (again, as I understand it) can occur when the wing isn't stiff enough tortionally to prevent twisting about the aircraft's lateral axis when a control input is made. Some old warplanes had this characteristic at high speeds.
For example, the pilot makes a control input to the left. The right aileron deflects downward, and instead of raising the overall AOA of the wing, it lowers it because the aileron acts like a trim tab, twisting the wing's leading edge downward.
Your question is asking: If the control reversal velocity is calculated to be higher than the divergence velocity, (When the wing breaks apart), how then can control reversal take place in flight?
The answer would be when it occurs near a stall.

The divergence speed isn't the speed where the wing breaks apart, it's the speed where drag begins to sharply increase due to transonic mach effects.

@Sean divergence speed in aeroelasticity (or aeroelastic divergence speed) is the speed above which the wings twist off. Divergence can have different meanings depending on the context.

At high speeds a large downward aileron deflection can cause the laminar flow to separate from the top of the wing and become turbulent. At this point the wing essentially ceases to produce lift, whereas the other wing has an upward aileron deflection and continues to produce lift. This rolls the aircraft in the opposite direction than intended. I have experienced this in a C152 aerobat while recovering from a steep dive, there is some buffeting and a slow roll opposite to the control input.
I have also heard that at trans-sonic speeds control inputs can precipitate shock waves on parts of the airframe that can cause control reversal without laminar flow separation. But I have not experienced this in a C152 (yet).

I sincerely hope you never experience trans-sonic speeds in a C152.

There is another effect that would have been the cause of the tail of the space shuttle to have a reverse effect above about mach 1.5, where the front of the tail is blunt, designed to deflect hot air during re entry. See diagram.

This is more of a comment than a full answer. Could you add some more information to answer the original question?


text:
What does "OBJECTIONABLE" indicate on a VFR chart?
On the Tulsa, Oklahoma VFR sectional the top of the outer Class C ring says "OBJECTIONABLE". Does this indicate some information on the chart is in dispute? I don't see anything in the VFR chart legend or side panels about the meaning.

It's highly unlikely that this designation has anything to do with Tulsa's class C. It's much more likely that it applies to one or more of the private fields (see here).

It means that the airspace around the airport is still under review after a proposed change.
This article explains it in more detail: http://expertaviator.com/2012/07/31/what-is-an-objectionable-airport/
And here's the official text: Part 157

§157.7   FAA determinations. (a) The FAA will conduct an aeronautical
  study of an airport proposal and, after consultations with interested
  persons, as appropriate, issue a determination to the proponent and
  advise those concerned of the FAA determination. The FAA will consider
  matters such as the effects the proposed action would have on existing
  or contemplated traffic patterns of neighboring airports; the effects
  the proposed action would have on the existing airspace structure and
  projected programs of the FAA; and the effects that existing or
  proposed manmade objects (on file with the FAA) and natural objects
  within the affected area would have on the airport proposal. While
  determinations consider the effects of the proposed action on the safe
  and efficient use of airspace by aircraft and the safety of persons
  and property on the ground, the determinations are only advisory.
  Except for an objectionable determination, each determination will
  contain a determination-void date to facilitate efficient planning of
  the use of the navigable airspace. A determination does not relieve
  the proponent of responsibility for compliance with any local law,
  ordinance or regulation, or state or other Federal regulation.
  Aeronautical studies and determinations will not consider
  environmental or land use compatibility impacts.
(b) An airport determination issued under this part will be one of the
  following:
(1) No objection.
(2) Conditional. A conditional determination will identify the
  objectionable aspects of a project or action and specify the
  conditions which must be met and sustained to preclude an
  objectionable determination.
(3) Objectionable. An objectionable determination will specify the
  FAA's reasons for issuing such a determination.
(c) Determination void date. All work or action for which notice is
  required by this sub-part must be completed by the determination void
  date. Unless otherwise extended, revised, or terminated, an FAA
  determination becomes invalid on the day specified as the
  determination void date. Interested persons may, at least 15 days in
  advance of the determination void date, petition the FAA official who
  issued the determination to:
(1) Revise the determination based on new facts that change the basis
  on which it was made; or
(2) Extend the determination void date. Determinations will be
  furnished to the proponent, aviation officials of the state concerned,
  and, when appropriate, local political bodies and other interested
  persons.

In this case the culprit is OK94 ("Sand Ridge Airpark"), which has been deemed "Objectionable" because it conflicts with approaches to Tulsa International.

I believe that your opening statement, It means that the airspace around the airport is still under review after a proposed change, is incorrect. I believe the label, OBJECTIONABLE, indicates that the FAA has made the airspace determination.

@JonathanWalters Not quite. It's more like a hint to what the FAA wants to do with the airspace and the public can object to that preliminary decision.

@voretaq7: Seems like they've worked out their problems, since Sand Ridge no longer shows up on the sectional as "OBJECTIONABLE".

According to the FAA Aeronautical Navigation Products FAQ:

What does "OBJECTIONABLE" stand for on VFR Charts?
"OBJECTIONABLE" indicates an airspace determination per FAA Joint
  Order 7400.2J Section 4, Airport Charting and Publication of Airport
  Data, issued 9 FEB 2012. When you see this indication on a chart be
  sure to refer to the applicable Airport/Facility Directory for more
  information. FAA Regional Airports Offices are responsible for
  airspace determinations. Address any challenges to objectionable
  airspace determinations to your regional airports office.

FAA Joint Order 7400.2J Section 4 refers to Airport Airspace Analysis and says (emphasis added by me):

10−1−1. PURPOSE
a. This part provides guidelines, procedures, and standards that
  supplement those contained in 14 CFR part 157, Notice of Construction,
  Alteration, Activation, and Deactivation of Airports.
b. These guidelines, procedures, and standards must be used in
  determining the effect construction, alteration, activation, or
  deactivation of an airport will have on the safe and efficient use of
  the navigable airspace by aircraft.
10−4−1. POLICY
a. All landing facilities which have received airspace determinations
  or those not analyzed, must be properly documented and processed in
  accordance with procedures contained in FAAO 5010.4, Airport Safety
  Data Program.
b. Landing facilities that have received objectionable airspace
  determinations must be published in the NFDD as “objectionable.” They
  must be depicted on VFR aeronautical charts only and without
  identifying text other than to designate objectionable status. They
  must not be published in the Airport/Facility Directory (A/FD).


text:
What are these "pimples" on the jet engine for?
When looking into the General Electric CJ 805-23B jet engine, I have discovered that all forward section of the compressor is covered by a number of near identically looking "pimples". What are these tiny devices for, what are they doing? And why only front section of the compressor has them?

They are the roots of the variable stator vanes that control the direction of the airstream as it enters each compressor stage to manage and optimize the angle of attack of the compressor blades in each stage.  The vanes are connected to the ring shaped gang bar and somewhere around the circumference there is an hydraulic actuator, or a series of them, that drives the gang bar rings to move all the vanes connected to it at the same time.  The hydraulic actuator is usually powered with pressurized fuel and is operated by the fuel control system to keep the vanes at the optimal rotational position.
This video (thanks, @Mackk) shows how the vanes operate:


text:
Is there a reason Germania aircraft cabins have blue lights?
On my 2014-01-18 CET flight ST2114 from FKB to LPA on a Germania airplane, cabin lights were switched from white to blue for takeoff. I recorded this in a panoramic photo.
Granted, on sunset, the sun coming in through the windows made up for a beautiful play of light: Blue and yellow are complementary colors.
Is there a security or technical reason for the blue light, or is it just for
aesthetics?

First thought on reading the title question was https://www.washingtonpost.com/news/to-your-health/wp/2018/06/29/the-plan-to-keep-drug-users-from-shooting-up-in-public-restrooms-and-why-it-may-be-a-bad-idea/    its probably unrelated though.

Bergische Universität Wuppertal has done some research that chronobiologically-adapted LED lighting can enhance passengers well being and they can sleep better during a flight.
This article details about this research:

Adaptive solid-state lighting (SSL) scenarios in which the color and
  brightness of lighting is varied during long-distance overnight
  flights results in passengers that sleep better and are more alert
  upon arrival.
... at the start of an overnight flight that warm-white light helps to
  stimulate melatonin production and increases passenger relaxation ...
  blue-rich cold light in the morning increased alertness

There have been studies showing benefits of biologically effective lighting for more relaxed flights.

+1 - Great answer and references, I remember reading a scientific article that mentioned night vision was no better or worse using red than any other color. Whatever color is used at night will diminish the eyes ability to see it for about 10min.


text:
Is it legal to bring (or allow someone else to bring) a firearm on a private flight?
Say that I want to leave the city and go hunting somewhere.  Is it legal to bring a hunting rifle and ammunition with me in a private airplane?
If yes, are there specific procedures to follow in order for it to be done safely?

My understanding is that the FAA takes no position on this, rather it's subject to local laws while you're on the ground.

I understand that Alaskan bushplanes ferry hunters around, most would bring hunting rifles, as long as they are stored correctly (unloaded) then I don't think they'll make a problem out of it

@ratchetfreak Alaska has a lot of strange rules/exceptions though so I'm not sure that I would use their procedures as evidence that something is legal in the rest of the US.

Prepared Pilot points out that firearms have not been required by the state of Alaska since 9/27/2001. Previously, they were.

This is a tricky question, because it depends greatly on the laws of the locations you're flying to / from.  Here's a few of the things you have to take into consideration.
Transporting is generally ok

According to 18 U.S.C. 926A:

Notwithstanding any other provision of any law or any rule or
regulation of a State or any political subdivision thereof, any person
who is not otherwise prohibited by this chapter from transporting,
shipping, or receiving a firearm shall be entitled to transport a
firearm for any lawful purpose from any place where he may lawfully
possess and carry such firearm to any other place where he may
lawfully possess and carry such firearm if, during such transportation
the firearm is unloaded, and neither the firearm nor any ammunition
being transported is readily accessible or is directly accessible from
the passenger compartment of such transporting vehicle: Provided, That
in the case of a vehicle without a compartment separate from the
driver's compartment the firearm or ammunition shall be contained in a
locked container other than the glove compartment or console.

926A says nothing about the method of transport.  Most often it applies to motor vehicles, but it can just as easily apply to aircraft.  Therefore as long as you're ok to carry at the departure airport and the destination airport, federal law protects you in between.
Also, remember that 926A only protects you if the firearm is unloaded and locked / inaccessible.
Ok, but can I bring my gun unloaded onto airport property, right?

Not in a sterile area
You can't take a firearm into the sterile area of an airport.  49 CFR 1540.5 defines a sterile area as:

Sterile area means a portion of an airport defined in the airport
security program that provides passengers access to boarding aircraft
and to which the access generally is controlled by TSA, or by an
aircraft operator under part 1544 of this chapter or a foreign air
carrier under part 1546 of this chapter, through the screening of
persons and property.

So, if the terminal and the GA ramp are two separate entities, you're probably ok.  However, I've flown to some smaller airports where the GA ramp and the TSA terminal are right next to each other.  In these cases, you need to be careful that you don't get into the sterile area with your firearm.
Maybe not anywhere on the airport
Local law might prohibit the possession of a firearm anywhere on airport property, or the airport itself might ban firearms on their premises.  The only way to know is to check the specific airport you intend to fly out of.
What if I want to carry loaded?

The first thing to remember if you carry loaded is that 18 USC 926A does not protect you.  In that case, you're exercising the privileges of either open or concealed carry according to the laws of the departure and destination states.
Open carry
Some states prohibit open carry.  Others allow open carry when unloaded.  Others allow open carry when loaded.  Some require a permit.  Some don't.  Some only require a permit for handguns but not long guns.  If you're going to open carry (perhaps a hunting rifle on a sling), make sure that the local area allows this.
Concealed carry
Some locations prohibit concealed carry outright (to the best of my knowledge, you still can't carry concealed in Chicago, for example).  Other states allow concealed carry with a permit from either that state or a state which they have reciprocity with.  So, for example, a Mississippi permit would be honored in Texas, but not in Nebraska.  Others allow concealed carry without a permit at all (that is, the 2nd Amendment is your permit).  First, be sure that your concealed permit has reciprocity with the destination state.
Even if you think you're open carrying, watch out.  In some states, the act of being in a vehicle, no matter how visibly your firearm is displayed, makes it concealed carry.  If you don't have a concealed carry permit, you may be in violation.
Is there anything else waiting out there to bite me?

Oh yes.
For example, Rhode Island.

A Rhode Island carry permit is generally required to
transport a handgun in a vehicle. This requirement does not apply,
however, to the holder of an out-of-state carry permit who intends to
travel uninterrupted through the state.

Is a fuel stop an "interruption"?
Or New Jersey.

[New Jersey] State law generally prohibits any person from knowingly possessing or
transporting any rifle or shotgun without first having obtained a New
Jersey firearms purchaser identification card (FPIC),or from
possessing or transporting any handgun without first having obtained a
New Jersey handgun carry permit.

Or Chicago.

Chicago, for example, bans handguns declared “unsafe” by the
Superintendent of the Department of Police, so-called “assault
weapons,” machine guns and other firearms regulated under the National
Firearms Act, “laser sight accessories,” firearm silencers or
“mufflers,” “high-capacity” (i.e., more than 12-round) magazines, and
“metal piercing bullets” (which include ammunition with non-lead
projectiles).  All these items are subject to seizure and forfeiture.
In some cases, the local ordinances have no explicit exceptions to
these bans for travelers merely passing through the jurisdiction

Or New York City.

Possession of a handgun within New York City requires a New York City
handgun license or a special permit from the city police commissioner
validating a state license within the city. Even New York state
licenses are generally not valid within New York City unless a
specific exemption applies, such as when the New York City police
commissioner has issued a special permit to the licensee or “the
firearms covered by such license are being transported by the licensee
in a locked container and the trip through the city of New York is
continuous and uninterrupted.”

Look, just tell me what to do.
Fine.

Carry your firearms unloaded, locked, and inaccessible.
Make as few intermediate stops as possible
Know and comply with all local laws both at your departure and your destination
Stay out of sterile areas while armed on airport property

So I can carry a loaded gun as PIC if it is legal (either open or concealed) in the state that I'm flying in?


text:
On SIGWX (significant weather) charts, what does **/** mean?
On SIGWX charts, it shows pairs of symbols with, say, */** or **/**. I know what the symbols mean on either side, but why are there two, and what does the slash indicate?
Would love good resources that explain more, too.
Example chart here, from the FAA sample questions (caution: 37 MB download), Figure 20, over Southern California.
I’m also interested in knowing what a dot with R underneath means.

http://www.beyourowncaptain.com/downloads/files/Reading%20Prog%20Charts.pdf
http://aviationweather.gov/static/adds/docs/metars/wxSymbols_anno1.pdf
Asterisks are snow, dots are rain. The slash means a mixture of the two.

The dot with an R under it is a substitution for the the thunderstorm symbol.

What you're looking at isn't a real SIGWX chart, it's a testing figure.

if it’s a mixture, then why do they show (actual example from chart) **/**? You can’t have a mixture of the same thing.

Can you link to the chart?

http://www.faa.gov/training_testing/testing/test_questions/media/sport_rec_private_akts.pdf (caution: 37 MB download), Figure 20, over Southern California

added a picture of the chart in my question

Honestly, I believe it's a mistake. I have found 3 sources confirming that means mix precipitation. One of them is the Aviation Weather Services advisory circular. You can see here, on page 8-3 http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC-0045G_chg1_fullDocument.pdf

Jungroth: hmmm, interesting! Thanks for digging out the source material. It’s a bit strange because the **/** occurs quite a few times in the chart!

@Peter I recall seeing **/** on my written test too - I can't think of any time where I've seen it on a real SIGWX chart though, and as I recall there were no questions in the question bank about the "mix of the same thing" precip areas (this might have changed though).


text:
How are take-off segments defined?
From take-off performance calculations I remember that the take-off and initial climb was expressed in a number of segments. Each segment had its own performance requirements. I presume they are described in some FAR / JAR but I don't know whether to look under certification or operational requirements.
What are the segments and how are they demarcated?
Do they differ for aircraft types (propeller vs. turbofan, number of engines?)
Are these segments used for certification, operationally or both?

You can find the 4 segment climb profile described for transport category airplanes in the (old) Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25A) Ch 10 and codified in 14 CFR 25.111.  The full 4 segment profile described below assumes an engine failure at $V_1$ and stipulates a climb and speed profile to clear obstacles.  During normal all-engine ops this profile is still technically in place but you'll hit your speed and altitude targets in such a way that you'll takeoff into a climb with increasing airspeed and no intermediate level off (you will still likely use parts of the profile for your flap retraction schedule).
The segments of the climb are:

Liftoff to 35' and $V_2$
At liftoff you are assumed to be in takeoff configuration with at least one engine not operating.  After liftoff you will retract the gear and accelerate to $V_2$ while climbing.  The goal here is the reach 35' height before the end of your takeoff distance.  Note that for propeller aircraft, this height is 50' instead of 35'.

35' to acceleration height flown at $V_2$
During this segment you climb at $V_2$ until you reach a height at which it is safe to level off and accelerate.  This height is called the acceleration height and is normally 400' AGL.  However, this value can vary per runway and will be available to you in your performance data for a particular runway.

Level flight at acceleration height to $V_{FS}$
You are clear of all obstacles, and now your goal shifts from altitude to airspeed.  Level the airplane and accelerate.

Continue climbing at $V_{FS}$ to 1500' (typical)
Once $V_{FS}$ is reached you will continue to climb at that airspeed.  This will also mark your transition from takeoff flaps to a clean configuration.  The height of 1500' is a typical value but may be different based on obstacles for a specific runway.

Enroute climb
Upon reaching 1500' (or whatever altitude is used in the previous segment) at $V_{FS}$ you transition into your enroute climb.  At this altitude you will begin to perform checklists for the engine failure you experienced between $V_1$ and liftoff and your after takeoff flows and checklists.

Fig 10-36 FAA-H-8083-25A pp 10-30
A transport category airplane with an engine inoperative must be able to meet the climb gradients stipulated in the image above.  The climb gradients vary by number of engines,

What a wonderful answer! Just a point: For propeller aircraft in the US, I think the certification requirement is to reach the best climb gradient speed V2 at 50 ft instead of 35. So the definition of the first climb segment is changed. Reference: 14 CFR Sec. G36.111: Flight Procedures.

What a wonderful answer! Just a point: For propeller aircraft in the US, I think the certification requirement is to reach the best climb gradient speed V2 at 50 ft instead of 35. So the definition of the first climb segment is changed. References: 14 CFR 121.177 - Airplanes: Reciprocating engine-powered: Takeoff limitations and 14 CFR Sec. G36.111: Flight Procedures

Hi, Could you explain what you exactly mean by "safe" in "During this segment you climb at V2 until you reach a height at which it is safe to level off"? I know this is a very stupid way of formulating things but, do you mean "not seeing any building or mountains" by safe?

@trilolil an altitude that guarantees sufficient clearance from all obstacles. This altitude will be specified for the runway if non standard.

@casey obstacles = buildings, mountains?

@trilolil buildings, trees, terrain, towers, tethered balloons, etc.


text:
What is the advantage of combining the rudder and brake pedals in aircraft?
I've read about several aircraft having their rudder and brake pedals very close together. To me this seems confusing and prone to error. It looks easy to mistake the rudder / brake pedals and inadvertently press the wrong one when flying or taxiing the aircraft. Since these are fundamentally important controls, this could lead to disaster.
So what is the advantage of keeping the rudder / brake pedals in this configuration? Are there any historical reasons for this design choice?

Note that the rudder and brakes are on the same pedals, not different ones.  Nothing to confuse.

There is at least one accident related to brake pedals, Crash: Yak Service YK42 at Yaroslavl on Sep 7th 2011, failed to climb on takeoff. The problem was not caused by the combining itself, but the fact that the pilots switched from different aircraft type with different pedal design and didn't get sufficient training (plus all the other holes in the cheese like poor take-off briefing).

All of the aircraft that I have ever seen have the rudder and braking functions combined into one set of two pedals.  To operate the rudder you press on the bottom part of the pedals, so that they slide back and forth on tracks, and to operate the brakes, you press the top part of the pedals so that they rotate towards the floor.  The left pedal operates the left wheel brake, and the right pedal operates the right brake, so you can use differential braking to turn the plane.  For rudder operation, the motion of the two pedals is locked together (i.e. press on the right pedal for right rudder, and the left pedal moves aft while the right pedal moves forward).
Because of the two different motions (rotation for brakes versus sliding for rudder) there isn't a problem with hitting the brakes when you want rudder or vice versa.  Typical early training is to operate the rudder using your toes on the lower parts of the pedals, so and to only put your whole foot on the pedal to actuate the braking mechanism.

In what aircraft does the rudder pedal assembly slide back and forth on tracks? Granted I don't have experience in all industry sectors, but all assemblies I have experience with are hinged, not tracked. The only tracked assembly I have come across is a PC flight simulator peripheral.

Here are a few design constraints:

Unlike a car, there is sometimes the need to apply the left brake independently from the right brake.  (See What is differential braking? for more details and pictures.)
There is a need to control the rudder, and you need to be able to move it either direction.
The pilot needs to be able to make small and rapid adjustments to them both, in both directions.
The pilot needs to be able to use both at the same time.
The pilot needs to be able to use both while at the same time controlling pitch and bank.

Based on this, it's pretty easy to see that a push button, lever, or toggle switch somewhere wouldn't meet all of these requirements, especially if you need to use your hands (which are already occupied with the yoke) to do it.
There are three axis of motion on an aircraft that the pilot can control directly, and with current designs, the flight controls move the aircraft in the same direction that they are moved:

Bank is controlled by turning the yoke left and right, which lowers the wing in the direction that you turn it.
Pitch is controlled by pushing the yoke forward or pulling it back, making the nose raise or drop.
Yaw is controlled by pushing the pedals in the direction that you want the nose to pivot.
Since your feet are already on the rudder pedals and you need two brake pedals too (point one from above), they usually just put the brakes on the top of the rudder pedals so that they can be used at the same time.1

1 There are also hand brakes in some older aircraft that require you to use your hand.  This has been pretty much abandoned in favor of toe (or heel) brakes since it did not allow for differential braking.

+1 just a footnote to your footnote: you get all sorts of mechanisms (handbrake, coupling with airbrakes, pedal brakes) in sailplanes, which usually feature one main wheel anyway.

There are a couple of newer LSAs that use hand brakes too, like the Dynamic WT9.

Many aircraft (esp. older Russian military designs, such as MiG-21) used a hand brake on the central stick and could do differential braking, by moving the rudder pedals the usual way. The pedals released the braking pressure from the respective wheel. In fact, these aircraft usually relied solely on differential braking for turns.

It is by design.
An airplane have three axes to move, unlike a car, thus more controls should be provided to the pilot. As primates have two legs and two arms, the designers of airplanes put foot pedals to control the rudders.
Because Newton came up with the first law of motion before airplanes were built, we needed to have devise a way to stop airplanes, and hence brakes were invented. But unlike cars, where a foot or feet are used to apply the brakes, rudder pedals were already occupying that region. And hence airplane designers added brakes on top of the rudder pedals.
Like everything else, this also evolved overtime.

The rudder's direction in aircraft since the "Golden Age" of flight
  between the two World Wars into the 21st century has been manipulated
  with the movement of a pair of foot pedals by the pilot, while during
  the pre-1919 era rudder control was most often operated with by a
  center-pivoted, solid "rudder bar" which usually had pedal and/or
  stirrup-like hardware on its ends to allow the pilot's feet to stay
  close to the ends of the bar's rear surface.

I disagree with the statement that combining them leads to any confusion. The rudder pedal/brake assembly is very easy to understand and master. The mistakes you make as a student are minimal and do not happen after a little training anyway.
Another reason for having rudder pedals on the floor is that in a coordinated turn, you might have to add slight rudder along with banking the airplane. As hands are used to bank, rudder pedals are controlled by feet. This situation is only applicable during flight.
During flight, brakes do not have any effect as they are applied on the main landing gear.
During taxiing, when you press the rudder pedal (not applying brakes), on smaller airplanes, the nose wheel is turned.

Just out of curiosity, have any notable aircraft been built that eliminate rudder pedals in favor of twisting a "joystick" (sidearm or central) or even the yoke? Or is that just too different from what pilots are used to? I believe that some spacecraft used twisted sidearm controllers for yaw control.

I can think of only linked aileron and rudder systems like in the Ercoupe. Interestingly, the Wright Flyer had it's yaw control linked to it's roll control (which was wing warping, not ailerons).

One other factor... this results in a more natural usage of the control. Since a lot of aircraft steer on the ground by differential braking, the pilot will use the same control and same basic thought process for yaw on the ground that they use in the air. In the air, left rudder to yaw left. On the ground, press on the top of the left rudder pedal to turn left. This is one less detail that a pilot has to keep in their mind. They have plenty of details already, so any reduction in pilot mind load is a big bonus.
Early aircraft, especially early airliners like the Ford Trimotor and DC2, used a braking bar to accomplish the differential braking (called the 'Johnson bar'... it was about .75 meters in length, so some wishful thinking was probably involved) that required a fair amount of skill to master, as well as a fair amount of arm strength. Hydraulic brakes on the rudder pedals were a huge improvement.


text:
How does stall risk in paramotors differ from that of conventional fixed-wing aircraft?
I happened to be walking past a small outdoor fair today, and saw someone flying a paramotor. The craft took off and started a steep ascending turn losing speed rapidly, seemed almost to come to a stop (relative the ground, but the flags nearby didn't show much wind, either) before the pilot brought it back to more level flight and got some way again.
I'm not a pilot, but I'm roughly aware of the mechanism of stalls and the consequent danger of high angle of attack, low speed maneuvers and I was briefly worried that there was going to be a problem.
In hindsight, the craft clearly has a very low stall speed (not much more than walking speed, I guess) but it did leave me wondering.
Are these craft are more or less at risk of stalling than conventional fixed wing aircraft?

And why should we actually care about stalling? Even without any lift, aren't we supposed to soar down gently, para(chute) way?

I suppose with paramotor you mean a powered paraglider, as pictured below.

Paramotor (picture source)
Since they obey the same laws of physics, they also stall, just like a fixed-wing aircraft does. There are, however, some peculiarities which should be mentioned:

The shape of the wing is produced by ram air filling the parafoil, so speed will help to keep the wing inflated and in shape. At low speed and in gusty winds the risk of the parafoil collapsing (severe deflation) is higher. This was the reason for several fatal accidents in the early times of paragliders, when the aspect ratio of high-performance parafoils was increased beyond safe limits. Today's designs will normally quickly recover from a deflation event, but not without a substantial loss of altitude.
The pressure forces on the parafoil will change its shape depending on its angle of attack. At low speed the angle of attack is higher, resulting in a suction peak close to the leading edge which in turn will increase the local camber of the foil. This leads to severe lift loss in a stall, and recovery needs some altitude. A fixed-wing aircraft, on the other hand, can employ an airfoil with only little lift loss in a stall, so recovery needs less altitude. This is by no means universal - there are also fixed-wing aircraft with less benign stall characteristics.
If the stall on a fixed wing starts near the wing tip, the aircraft might enter an uncontrollable roll. This can happen in steep turns and with badly designed aircraft. This is not possible in a parafoil; it might instead slide sideways with a partially collapsed parafoil, but stay upright due to its low center of gravity. If the pilot panics and pulls his brake lines on one side during a stall, the parafoil can enter a spin. Similarly, if the pilot of a fixed-wing aircraft insists on applying rudder when stalled, the aircraft can also enter a spin.

Generally, due to their low wing loading and the drag of the raiser lines, paramotors fly at much lower speeds than aircraft, so accidents involve less energy, but the pilot has no protection around her/him, so fatal accidents do occur more regularly than with fixed-wing aircraft. For paragliders, Wikipedia gives a number of 2 fatalities per year for 10.000 pilots.

But how bad it is when it stalls? Since parachutes rely purely on post-stall lift and paraglides are not much smaller I would expect the remaining lift after stall to still be enough to keep the rate of descent in survivable range.

Paragliders might also fly more often, so I'm not sure the statistic is comparable

@JanHudec: Good fixed-wing airfoils have the same lift coefficient for several degrees of AoA into the stall. Parafoils go from approx. 1.5 right before stall to 1.1 with separated flow, and their effective area shrinks because the parafoil starts to collapse. Stall speed (forward) is slightly less than the vertical speed when used as a parachute, so a couple of meters are needed to accelerate downwards. When dropping in parachute mode, a paramotor pilot will get hurt and might kill himself if he is unlucky.

fatal accidents do occur more regularly than with fixed-wing aircraft... I disagree. Do you have a source?

@Cloud: Yes. Not only Wikipedia (see last line of the answer), but also statistics for paragliders and fixed wing aircraft. Unfortunately, not normalized to flight hours, but 34 accidents with severe injuries or death with fixed wing aircraft versus 139 with paragliders support my position.